1. Field of the Invention
The present invention is directed to systems and methods for measurements of wireline cable and coiled tubing as they are used in the oil or gas industry; and, in certain particular aspects, to non-contact measurement (e.g. of depth and velocity) of wireline cable and coiled tubing.
2. Description of Related Art
There are a variety of known methods used to measure the depth of equipment within oil and gas wells. In many instances these methods employ devices that are in physical contact with a conducting mechanism that transports the equipment through the wellbore, e.g. on a wireline cable (“wireline”) or with coiled tubing. For example, often, when a wireline or coiled tubing operation is in progress, one known method of depth measurement relies on either one or two measurement wheels that are in contact with the wireline or coiled tubing that are in turn connected to quadrature encoders. The quadrature encoders produce a series of pulses as the measurement wheel(s) rotate in concert with the wireline cable or coiled tubing motion. At any particular measurement wheel the number of pulses is proportional to the length of wire or tubing that passed the measurement wheel. Several of such known depth measurement systems suffer accuracy and repeatability problems that manifest themselves as an inability to correctly record the depth of a leading end of a wireline cable or coiled tubing in the wellbore. Repeatability problems are often caused by an undetermined amount of slip which results when cable or tubing passes measurement wheels without an equivalent rotation of the measurement wheels being recorded.
Repeatability errors often are due to the inability of existing depth measurement systems to measure the amount of stretch in a cable or tubing. Stretch is caused by the weight of the equipment attached to the cable or tubing and the weight of the deployed cable or tubing. The stretch length can also depend on other factors such as friction and wellbore deviations.
In many known systems, accuracy is dependent on the diameter of the measurement wheels being known to a specified tolerance. Measurement wheels are prone to collecting dirt and grease that result in a change in an effective diameter. Although the diameter change is small, e.g. over a long 10,000 foot length of cable or tubing, the cumulative error in measurement can be significant and undetected. Over a prolonged period, wear of the measurement wheels is also factor.
An exemplary known wireline unit is a commercially available Wireline unit L▴6901001933 manufactured by Elmar, a National Oilwell Varco company located in Houston, Tex. (and co-owned with the present invention).
Existing exemplary coiled tubing units are the known coiled tubing units MKT10T or MK20T manufactured by Hydra Rig, a National Oilwell Varco company located in Dallas, Tex. (and co-owned with the present invention).